Amazon.com founder Jeff Bezos was one of the biggest dollar gainers on this year's Forbes 400, adding $6.5 billion to his fortune over the past year -- and managing to stay just ahead of Mark Zuckerberg, who is hot on his heels.

Bezos' boost comes on the back of an impressive 55% rise in Amazon shares, pegging his net worth at $19.1 billion. He’s now the 13th richest person in the U.S., just behind New York's Mayor Mike Bloomberg. He's just one spot ahead of young pretender to the tech throne Zuckerberg, whose net worth increased a staggering $10.6 billion since August 2010 to a current $17.5 billion as the private market valuation of Facebook nearly tripled to $66.5 billion.

Dot com king Bezos needn't be too worried about Zuckerberg just yet, though. Once just an online bookseller, Amazon is now the world's largest internet retailer of any kind. Its digital book business is booming: as of April, Amazon has sold 105 e-books for every 100 printed books that left its warehouse. Its Kindle device is hugely popular, with over 17 million sold to date. That number looks to grow with news that libraries may start to lend books out on the e-reader.

Bezos remains staunchly private, rarely speaking publicly and not giving in to speculation about Amazon's plans, or his own. He's stayed mum on persistent rumors that Amazon is set to launch a tablet to compete with the Apple iPad. His reticence, of course, only provides grist for tech bloggers and keeps Amazon acolytes coming back to check for word on the mystery device.

He's also kept quiet about his other big investment: Blue Origin, a highly secretive commercial space company he funds. Bezos has been funneling money into the Texas firm for years in hopes of developing a vertical take-off, vertical landing rocket. He broke with protocol and spoke out in early September when one of Blue Origin's spacecraft crashed during a test flight. He issued a rare statement on Blue Origin's blog only after locals told Forbes of the explosion, writing:

“Three months ago, we successfully flew our second test vehicle in a short hop mission, and then last week we lost the vehicle during a developmental test at Mach 1.2 and an altitude of 45,000 feet,” said Bezos in the post. “A flight instability drove an angle of attack that triggered our range safety system to terminate thrust on the vehicle. Not the outcome any of us wanted, but we’re signed up for this to be hard, and the Blue Origin team is doing an outstanding job. We’re already working on our next development vehicle.”

As Blue Origin works on its next development, so does Amazon -- we presume, anyway. Bezos is one billionaire who can keep a secret.